Add ChannelLayout API (#38)

* Create channel_layout directory

* Rename ChannelLayout -> ChannelLayoutMask

* Add enum Channel (<-> AVChannel)

* Add struct ChannelCustom (<-> AVChannelCustom)

* Add enum ChannelOrder (<-> AVChannelOrder)

* Add struct ChannelLayout

- Smart copy-on-write pointer to AVChannelLayout
- idiomatic Rust API wrapping around FFmpeg APIs
- no Ambisonic support (yet)
- consts will need to be manually updated

* Add ChannelLayoutIter (iterator over all standard layouts)

* Add codec/Audio::ch_layouts

* Add ch_layout API to Audio-related structs

src/filter/context/context.rs

@@ -2,7 +2,7 @@ use std::marker::PhantomData;
 
 use super::{Sink, Source};
 use crate::ffi::*;
-use crate::{format, option, ChannelLayout};
+use crate::{format, option, ChannelLayoutMask};
 use libc::c_void;
 
 pub struct Context<'a> {
@@ -49,7 +49,7 @@ impl<'a> Context<'a> {
         let _ = option::Settable::set(self, "sample_rates", &i64::from(value));
     }
 
-    pub fn set_channel_layout(&mut self, value: ChannelLayout) {
+    pub fn set_channel_layout(&mut self, value: ChannelLayoutMask) {
         let _ = option::Settable::set(self, "channel_layouts", &value.bits());
     }
 }